[PATCH 01 of 14 V3] util: move 'readexactly' in the util module

[Gregory Szorc]

On Sat, Jan 20, 2018 at 12:22 PM, Gregory Szorc <gregory.szorc at gmail.com>
wrote:

> On Fri, Jan 19, 2018 at 3:47 PM, Boris Feld <boris.feld at octobus.net>
> wrote:
>
>> # HG changeset patch
>> # User Boris Feld <boris.feld at octobus.net>
>> # Date 1516391495 -3600
>> #      Fri Jan 19 20:51:35 2018 +0100
>> # Node ID 15f7795f96a5f9acb3ed2e640fcec82f3ccd6f53
>> # Parent  de32acb24949c0e3633de373d1c6c8c814faa804
>> # EXP-Topic b2-stream
>> # Available At https://bitbucket.org/octobus/mercurial-devel/
>> #              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r
>> 15f7795f96a5
>> util: move 'readexactly' in the util module
>>
>
> I was swamped the last few days and didn't have time to look at this
> series even though I really wanted to. I reviewed it this morning and am
> generally happy with it. However, I'd like to make some BC changes to the
> wire protocol so we don't ship things in the wire protocol that we'll need
> to support indefinitely. A number of other changes to make this a more
> useful feature can be done post RC, during freeze.
>
> *High priority issues* (should be addressed before RC)
>
> HTTP server is sending bundle2 response with compression. That's a
> significant reason why this format is slower than the old format. I also
> think bundle2 overhead is coming into play on the server. I need to profile
> once compression is removed from the equation. I'm pretty sure this change
> can be done during freeze since it requires the server to drop compression
> for this bundle2 response. But I'm not 100% about that given how
> compression is negotiated between client and server. Would prefer this
> change make it in before RC.
>
> The new "stream" bundle2 part has a "version" parameter. I don't like
> versioning bundle2 parts. I think the behavior of bundle2 parts should be
> consistent over time. If we need to introduce a new version of something or
> change processing semantics, we should be introducing a new bundle2 part
> and associate new/different behavior with the part. Attempting content
> negotiation within a named bundle2 part is extra complication and will lead
> to increased protocol chaos down the road. This should be changed before RC
> since it impacts the wire protocol.
>

First, I should add that I'm working on patches to fix at least the
higher-priority issues. I should have them sent in an hour or two.

Related to the bundle2 part and capabilities exchange, the existing code
isn't future compatibility because the client doesn't currently advertise
its support for stream versions to the server unless a config option is
set. Right now, it must be implied that if a client sends stream=1 to
getbundle without sending a client capability, they support at most the
"stream" part with "version=v2". The client should always advertise its
part/version support so a future server knows which version to send back to
the client. It isn't critical this be addressed before RC. But leaving it
as is makes the protocol less explicit and more difficult to reason about.
I'll attempt a patch.


>
> Requirements string in bundle2 part should ideally use the same
> pre-encoded normalization as bundlespecs for consistency with the rest of
> Mercurial. Since it touches the wire protocol, this needs to change before
> RC.
>
> *Medium priority issues* (ideally addressed before RC)
>
> I don't think there's an easy way to generate bundle2 bundles with stream
> parts. We need this to support clonebundles. This will likely require
> bundlespec changes. I would highly prefer to get this in before RC so we
> can do end-to-end integration testing with clonebundles.
>
> *Lesser priority issues* (can be fixed post RC)
>
> streamclone._emit() should be renamed to _emitv2() for consistency with
> other functions in file.
>
> I think streamclone.applybundlev2() should handle writing requirements,
> not the bundle2 part handler. Similar logic is already in streamclone.
> maybeperformlegacystreamclone().
>
> I need to look at this in more detail, but 133a678673cb ("allow bundle'2
> stream clone") is a bit suspect. It's correctness may depend on your
> interpretation of whether server.disablefullbundle applies only to
> changegroup bundles. I can't recall what existing behavior is with regards
> to stream clone. Need to double check.
>
> test-clone-uncompressed.t wants a comment about legacy stream clone phases
> being buggy. We should also decide what we want to do about issue #5648
> since bundle2 stream clone solves the problem. I'm tempted to mark #5648 as
> resolved once bundle2 stream clone is enabled by default.
>
> In 56c30b31afbe:
> @@ -1465,6 +1465,7 @@ def _pullbundle2(pullop):
>          kwargs['cg'] = False
>          kwargs['stream'] = True
>          pullop.stepsdone.add('changegroup')
> +        pullop.stepsdone.add('phases')
>
> Do we know for sure that phases were processed? It depends on what the
> server sent, no? I think this code may assume the behavior of the official
> Mercurial server and may not be compatible with other implementation.
>
> The use of temporary files for non-append-only files and the context
> manager and copy primitive associated with that code feels overly complex.
> Can we just slurp file content into memory? We already store the filenames
> and offsets in memory. We already scale memory use linearly with number of
> files in the repo. I don't see a huge problem doing the same for files that
> are proportional to number of changesets, branches, tags, and heads. That
> being said, the code is already landed. Some maybe we just keep it around.
> If we do keep it around, I think it could use some refactoring to improve
> readability. The amount of functions being called is a bit much for what
> the code is doing, IMO.
>
> The code around managing multiple vfs instances feels a bit complex. I
> don't think we need a dict of vfs instances. We only have 2. Let's inline
> their usage and do away with all the context manager and dictionary lookup
> complications.
>
> Related to vfs, we're not using the "expectedcount" argument with
> vfs.backgroundclose(). Ideally, the part header defines the number of files
> in each vfs so we can pass the proper value. But, I think it's fine to use
> the total expected file count as the hint to the store vfs and don't use
> background close for the cache vfs (since it will almost certainly never
> have enough entries to warrant background closing).
>
> I'm pretty happy with the feature. It is currently behind an experimental
> config flag. I think we can make it enabled by default in 4.5 if we fix the
> more important issues.
>
> *Other observations *
>
> The new format seems to use less CPU on the client than the old format
> when doing HTTP exchange. 30-31s versus 25-26s for mozilla-unified. And
> that's with zstd in play. I haven't profiled, but this is likely due to not
> using readline() and transferring caches and phases data (I believe we
> avoid a linear changelog scan post transfer now since I believe something
> before needed to reconstruct the branch cache).
>
> Here are some example numbers on my i7-6700K.
>
> legacy http
> server:
> 8.44user 3.51system 0:51.21elapsed 23%CPU (0avgtext+0avgdata
> 171184maxresident)k
> 1144inputs+0outputs (1major+73642minor)pagefaults 0swaps
>
> client:
> 30.79user 8.47system 0:42.47elapsed 92%CPU (0avgtext+0avgdata
> 197244maxresident)k
> 19288inputs+7039984outputs (2major+94838minor)pagefaults 0swaps
>
> v2 http
> server:
> 37.69user 8.01system 1:56.61elapsed 39%CPU (0avgtext+0avgdata
> 236808maxresident)k
> 6836840inputs+21288outputs (2major+82612minor)pagefaults 0swaps
>
> client:
> 25.82user 8.32system 1:47.83elapsed 31%CPU (0avgtext+0avgdata
> 209164maxresident)k
> 60848inputs+7061216outputs (2major+78863minor)pagefaults 0swaps
>
>
>>
>> This function is used in multiple place, having it in util would be
>> better.
>> (existing caller will be migrated in another series)
>>
>> diff --git a/mercurial/changegroup.py b/mercurial/changegroup.py
>> --- a/mercurial/changegroup.py
>> +++ b/mercurial/changegroup.py
>> @@ -32,14 +32,7 @@ from . import (
>>  _CHANGEGROUPV2_DELTA_HEADER = "20s20s20s20s20s"
>>  _CHANGEGROUPV3_DELTA_HEADER = ">20s20s20s20s20sH"
>>
>> -def readexactly(stream, n):
>> -    '''read n bytes from stream.read and abort if less was available'''
>> -    s = stream.read(n)
>> -    if len(s) < n:
>> -        raise error.Abort(_("stream ended unexpectedly"
>> -                           " (got %d bytes, expected %d)")
>> -                          % (len(s), n))
>> -    return s
>> +readexactly = util.readexactly
>>
>>  def getchunk(stream):
>>      """return the next chunk from stream as a string"""
>> diff --git a/mercurial/util.py b/mercurial/util.py
>> --- a/mercurial/util.py
>> +++ b/mercurial/util.py
>> @@ -3865,3 +3865,12 @@ def safename(f, tag, ctx, others=None):
>>          fn = '%s~%s~%s' % (f, tag, n)
>>          if fn not in ctx and fn not in others:
>>              return fn
>> +
>> +def readexactly(stream, n):
>> +    '''read n bytes from stream.read and abort if less was available'''
>> +    s = stream.read(n)
>> +    if len(s) < n:
>> +        raise error.Abort(_("stream ended unexpectedly"
>> +                           " (got %d bytes, expected %d)")
>> +                          % (len(s), n))
>> +    return s
>>
>
>
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